CHEMISTRY GLOSSARY

relative humidity    →   relativna vlažnost

Relative humidity is the ratio of the partial pressure of water vapour in air to the saturation vapour pressure of water at the same temperature, expressed as a percentage.

reverse osmosis    →   reverzna osmoza

Reverse osmosis is the method used for obtaining freshwater from saltwater. The process uses a semi-permeable membrane through which pure water and not the salts will pass. The saltwater must be pressurised to approximately 25 bar, which makes it operationally expensive to produce large quantities of fresh water by this method.

stp    →   stp

STP (or NTP) is abbreviation for standard (normal) temperature (273.15 K or 0 °C) and pressure (10⁵ Pa). These conditions are used when comparing the properties of gases.

universal gas constant    →   univerzalna plinska konstanta

Universal gas constant R has the value of 8.314 472(15) J K^-1 mol^-1. It corresponds to the volume work performed by one mole of gas heated by 1 K at standard pressure.

vacuum distillation    →   vakuum destilacija

Vacuum distillation is distillation under reduced pressure. The depression in the boiling point of the substance distilled means that the temperature is lower, which may prevent the substance from decomposing.

Gibbs free energy    →   Gibbsova slobodna energija

Gibbs free energy (G) is an important function in chemical thermodynamics, defined by

where H is the enthalpy, S the entropy, and T the thermodynamic temperature. Gibbs free energy is the energy liberated or absorbed in a reversible process at constant pressure and constant temperature. Sometimes called Gibbs energy and, in older literature, simply free energy.

Changes in Gibbs free energy, ΔG, are useful in indicating the conditions under which a chemical reaction will occur. If ΔG is negative the reaction will proceed spontaneously to equilibrium. In equilibrium position ΔG = 0.

Haber process    →   Haberov proces

Haber process is an industrial process for producing ammonia by reaction of nitrogen with hydrogen:

The reaction is reversible and exothermic, so that a high yield of ammonia is favoured by low temperature. However, the rate of reaction would be too slow for equilibrium to be reached at normal temperatures, so an optimum temperature of about 450 °C is used, with a catalyst of iron containing potassium aluminium oxide promoters. The higher the pressure the greater the yield, although there are technical difficulties in using very high pressures. A pressure of about 250 atmospheres is commonly employed. The removal of ammonia from the batch as soon as it is formed ensures that an equilibrium favouring product formation is maintained. The nitrogen is obtained from air. Formerly, the hydrogen was from water gas and the water-gas shift reaction (the Bosch process) but now the raw material (called synthesis gas) is obtained by steam reforming natural gas.

The process is of immense importance for the fixation of nitrogen for fertilisers and explosives. It was developed in 1908 by German chemist Fritz Haber (1868-1934) and was developed for industrial use by Carl Bosch (1874-1940), hence the alternative name Haber-Bosch process.

heat capacity    →   toplinski kapacitet

Heat capacity is defined in general as dQ/dT, where dQ is the amount of heat that must be added to a system to increase its temperature by a small amount dT. The heat capacity at a constant pressure is C_p = (∂H/∂T)_p; that at a constant volume is C_V = (∂E/∂T)_V, where H is enthalpy, E is internal energy, p is pressure, V is volume, and T is temperature. An upper case C normally indicates the molar heat capacity, while a lower case c is used for the specific (per unit mass) heat capacity.

vapour    →   para

Vapour is the gaseous state of some matter which can be converted into liquid when exposed to pressure.

volume    →   volumen

Volume is a space taken by a certain mass of some matter. SI unit for volume is a cubic meter (m³). Changes of temperature or pressure can change volume.